Currently, nuclear power plants produce large quantities of radioactive spent fuel. These nuclear power plants were originally constructed with a limited amount of storage space for the spent fuel, since it was originally contemplated that the spent fuel would be either reprocessed to provide fissionable uranium and plutonium or stored by the United States government at remote locations from the nuclear power plant. However, the spent fuel has not been reprocessed or removed from the nuclear power plants. Accordingly, the nuclear power plants are continuously seeking ways to better utilize the spent fuel storage space currently available at the plant in a way to permit the storage of larger quantities of fuel in the same given area. Thus, there exists a need to increase the storage capacity of the existing storage facilities.
Most current fuel assemblies are composed of a plurality of fuel rods containing nuclear fuel held in the fuel assembly. The fuel assembly typically has a square cross-section with a width ranging from about 6" to about 12" and a length of about 16 feet. The fuel assembly is used in the nuclear reactor, they are removed and stored in spent fuel storage racks. The storage racks have a plurality of vertically arranged cells designed to adequately support the fuel assemblies also during potential seismic events.
Spent fuel retains a measure of reactivity, i.e., neutron emissivity, which is appreciable but insufficient for economic use in a reactor. Accordingly, it is necessary that the spent fuel be stored in such a way that the mass stored does not become critical. In refueling a reactor, the fuel assemblies in specified areas of the reactor are replaced at intervals of several years. The residual reactivity of the removed or spent fuel assemblies throughout each area of the refueling is not uniform. It is then necessary, in the storage of spent fuel, to preclude nuclear criticality by reason of the presence of fuel assemblies having high residual reactivity.
Accordingly, prior art storage racks precluded such criticality by providing racks whose cells are appropriately spaced apart. In addition, quantities of neutron-absorbing material or poison can be provided in the cells of the racks in which the spent fuel is stored. The first of these solutions, i.e., spacing the cells apart, requires that the volume occupied by the storage racks be unreasonably large. The second solution, i.e., using poison, introduces a high cost factor.
Specifically, storage racks were originally comprised of tubes of a square cross-section with a fuel assembly placed in each of the tubes or cells. These tubes were connected together in batteries of several hundred tubes and placed in a pool of water or coolant to reduce the radiation effects. The tubes were connected to each other by welding the tubes in the upper and lower section to form square pattern of cells. The space between the tubes were used to place an additional radiation moderating material technically known as poison.
The racks with fuel assemblies in individual tubes, however, occupy significant space in the pool and their cost was high because each fuel assembly was surrounded by the tube material and therefore there were two metal walls of tubes between the neighboring two stored assemblies.
In order to place more cells in the pool as well as to reduce the cost of the racks, nuclear utilities began to replace the existing structures in the storage pools with racks of high density structure where only every second fuel assembly was placed in individual tubes. An example of such a storage rack is disclosed in U.S. Pat. No. 4,960,560 to Machado et al, of which the entire disclosure is hereby incorporated herein by reference.
By placing the tubes in a checkerboard pattern as disclosed in U.S. Pat. No. 4,960,560, it became possible to support every other fuel assembly in the space generated by the neighboring tubes. Thus, the area of the tube cells was reduced to about half, and the fuel assemblies could be placed closer to each other. The tubes of this storage were connected by weldments at one side of the tube corners and were attached to the bottom plate by fillet welding. Such asymmetric joints in such storage racks result in deterioration and reduces the accuracy of fabrication of the storage rack. Accordingly, such storage racks require larger tolerances to fabricate.
Examples of some other prior storage racks for storing spent nuclear fuel assemblies are disclosed in the following U.S. Pat. Nos.: 5,196,161 to Lewis; 5,032,348 to Blum et al; 4,948,553 to Machado et al; 4,900,505 to Machado et al; 4,857,263 to Cooney et al; 4,820,472 to Booker et al; 4,788,030 to Bosshard; 4,746,487 to Wachter; 4,710,342 to Grenon et al; 4,695,424 to Flynn; 4,630,738 to Bosshard; 4,400,344 to Robbins et al; 4,366,115 to Schlumpf; 4,319,960 to Larson et al; 4,288,699 to Boucherie et al; 4,287,426 to Anthony; 4,243,889 to Weber; 4,233,518 to Auyeung et al; 4,187,433 to Zezza; 4,177,386 to Robbins et al.; and 4,143,276 to Mollon.
In view of the above, it is apparent that there exists a need in for a storage rack for storing spent nuclear fuel assemblies which will overcome the above problems, and which utilizes less space than the prior storage racks, and which is relatively simple to manufacture and inexpensive to construct.
This invention these needs in the art along with other needs which will become apparent to those skilled in the art once given this disclosure.